Reinforcement steel

ABSTRACT

A reinforcement steel bar includes a shaft part extending in a front-back direction, and a head part formed by forging an end portion of the shaft part. A width in a right-left direction of an upper end portion of the head part is formed wider than a diameter of the shaft part. The width in the right-left direction of a lower end portion of the head part is formed narrower than the diameter of the shaft part. An upper end surface extending parallel to an axial direction of the shaft part is formed at the upper end portion of the head part. A left side surface and a right side surface of the head part are inclined such that the width in the right-left direction of the head part is gradually reduced from the upper end portion to the lower end portion of the head part.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to reinforcement steel.

2. Description of the Related Art

A coupling structure for coupling decks laid on a bridge superstructure is configured such that a reinforcement steel bar of one of the decks and a reinforcement steel bar of the other deck are caused to project to a space between the decks, and concrete is placed in the space.

There is an example of reinforcement steel used in a ferroconcrete structure such as the aforementioned deck coupling structure, in which a diameter of a head part of reinforcement steel is made larger than that of a shaft part thereof to increase an anchoring force to concrete.

A ferroconcrete structure is subject to the regulation of a covering depth of concrete from reinforcement steel to an outer surface of the structure. Regarding the reinforcement steel with the head part having the larger diameter than that of the shaft part as mentioned above, if the covering depth of the head part is adjusted to a regulated value, then the covering depth of the shaft part becomes larger than the regulated value. Hence, this results in an increase in weight of the structure.

In this regard, there is another example of the conventional reinforcement steel in which a head part is formed into a semicircular shape by forming a flat surface on the head part (see Japanese Utility Model Registration No. 3191370, for example). In this configuration, if the flat surface of the head part is directed to an outer surface of the structure, the covering depth of the flat surface of the head part is subject to the regulation of the covering depth. As a consequence, it is possible to control the covering depth of the entire reinforcement steel.

SUMMARY OF THE INVENTION

When the head part has the semicircular shape as described above regarding the conventional reinforcement steel, an amount of deformation of an end portion of the shaft part is increased when the head part is formed by forging the end portion of the shaft part. This leads to a difficulty in forming the head part.

An object of the present invention is to solve the aforementioned problem by providing reinforcement steel which is capable of increasing an anchoring force to concrete, controlling a covering depth of the concrete, and also facilitating formation of a head part.

To solve the problem, the present invention provides a reinforcement steel which includes a shaft part extending in a front-back direction, and a head part formed by forging an end portion of the shaft part. A width in a right-left direction of an upper end portion of the head part is formed wider than a diameter of the shaft part, while the width in the right-left direction of a lower end portion of the head part is formed narrower than the diameter of the shaft part. An end surface extending parallel to an axial direction of the shaft part is formed at the upper end portion of the head part. A left side surface and a right side surface of the head part are inclined such that the width in the right-left direction of the head part is gradually reduced from the upper end portion to the lower end portion of the head part.

Note that in the present invention, the expressions up and down, back and front, and right and left are defined for the convenience of clarifying a configuration of reinforcement steel, and are not intended to limit the structure and a mode of use of the reinforcement steel of the present invention. For instance, the upper end portion of the head part may be placed downward or sideways.

According to the present invention, when a stress attributable to a bending tensile force and to a punching shear force is applied to the reinforcement steel buried in concrete, the head part engages with the concrete. Thus, it is possible to increase an anchoring force to the concrete.

Here, the anchoring force to the concrete can further be increased when the above-described reinforcement steel is deformed reinforcement steel provided with ribs on an outer peripheral surface of the shaft part.

Meanwhile, when the reinforcement steel of the present invention is laid in a ferroconcrete structure, if the end surface of the head part is directed to an outer surface of the structure, then a covering depth of the end surface of the head part is subject to the regulation. Moreover, the covering depth of the end surface of the head part becomes substantially the same as a covering depth of the shaft part. As a consequence, it is possible to control the covering depth of the entire reinforcement steel.

In this way, it is possible to reduce the covering depth of the entire reinforcement steel and thus to reduce the weight of the structure. When the reinforcement steel of the present invention is applied to a deck, it is possible to increase strength of the deck while reducing its weight as low as that of the existing deck. Furthermore, the thickness of the deck can be kept at a minimum deck thickness as defined in design standards (the Specifications for Highway Bridges).

Meanwhile, in the reinforcement steel of the present invention, the head part is provided with the left side surface and the right side surface. Hence, the head part is formed into a substantially triangular shape. In this way, a volume of the head part can be reduced more than that in a configuration of forming the head part into a semicircular shape. This makes it possible to reduce an amount of deformation of the end portion of the shaft part so as not to cut off metallic fibers (fiber flows) of the head part when the head part is formed by forging the end portion of the shaft part. As a consequence, according to the reinforcement steel of the present invention, the head part can be formed easily.

In the above-described reinforcement steel, it is preferable to form each of the left side surface and the right surface into a flat surface so as to facilitate the formation of the head part.

In the above-described reinforcement steel, when a plate-shaped flange portion is formed at an outer peripheral part of a base end portion of the head part, it is possible to surely bring the head part into engagement with the concrete, and thus to increase the anchoring force of the reinforcement steel to the concrete.

In the above-described reinforcement steel, it is possible to increase a surface area of the head part when a protrusion that extends linearly on an outer surface of the head part is caused to project therefrom. Thus, the anchoring force of the reinforcement steel to the concrete can be increased.

In the above-described reinforcement steel, it is preferable to incline the end surface downward from a central portion in the right-left direction to a side edge portion.

In this configuration, if the end surface of the head part is slightly tilted about the axis of the shaft part when the reinforcement steel is laid in the ferroconcrete structure, it is still possible to control the covering depth of the end surface of the head part.

In the above-described reinforcement steel, it is preferable to provide a front end portion of the head part with a front end surface with its normal direction aligned with the axial direction of the shaft part, and to locate an outer edge portion of the front end surface outside, in a radial direction of the shaft part, of a corner portion between a base end surface of the head part and an outer peripheral surface of the shaft part.

In this configuration, the thickness from the front end surface to the corner portion between the base end surface of the head part and the outer peripheral surface of the shaft part is equal to a maximum value of a thickness of the head part in the axial direction of the shaft part. Thus, it is possible to increase shear strength of the head part when a tensile force is applied from the concrete to the reinforcement steel.

In the above-described reinforcement steel, if the corner portion is formed into a curved surface or if a recess is formed along the corner portion, a stress is less likely to be concentrated on the corner portion between the base end surface of the head part and the outer peripheral surface of the shaft part when a pressure originating from the concrete is applied from the base end side of the reinforcement steel to the front end side thereof. Thus, it is possible to increase fatigue resistance at a junction between the head part and the shaft part.

The reinforcement steel of the present invention is capable of increasing the anchoring force to the concrete and controlling the covering depth of the concrete. In addition, according to the reinforcement steel of the present invention, it is possible to reduce the amount of deformation of the end portion of the shaft part when the head part is formed. Thus, the head part can be formed easily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing reinforcement steel according to a first embodiment of the present invention.

FIG. 2 is a side view showing the reinforcement steel according to the first embodiment of the present invention.

FIG. 3 is a front view showing the reinforcement steel according to the first embodiment of the present invention.

FIG. 4 is a transverse sectional view showing a coupling structure using the reinforcement steel according to the first embodiment of the present invention.

FIG. 5 is a front view showing a modified example of the reinforcement steel according to the first embodiment of the present invention having a configuration in which an upper end surface of a head part is a flat surface.

FIG. 6 is a front view showing another modified example of the reinforcement steel according to the first embodiment of the present invention having a configuration in which a left side surface and a right side surfaced are curved.

FIG. 7 is a perspective view showing reinforcement steel according to a second embodiment of the present invention.

FIG. 8 is a side view showing the reinforcement steel according to the second embodiment of the present invention.

FIG. 9 is a front view showing the reinforcement steel according to the second embodiment of the present invention.

FIG. 10 is a front view showing a modified example of the reinforcement steel according to the second embodiment of the present invention having a configuration in which an upper end surface of a head part is a flat surface.

FIG. 11 is a front view showing another modified example of the reinforcement steel according to the second embodiment of the present invention having a configuration in which a left side surface and a right side surfaced are curved.

FIG. 12 is aside view showing reinforcement steel according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

Note that in the description of the embodiments, the same constituents are denoted by the same reference numerals and overlapping explanations thereof will be omitted.

Moreover, in the following description, the expressions up and down, back and front, and right and left are defined for the convenience of clarifying a configuration of reinforcement steel of each of the embodiments, and are not intended to limit the structure and a mode of use of the reinforcement steel of the present invention.

First Embodiment

As shown in FIG. 1, a reinforcement steel bar 1A of a first embodiment is deformed reinforcement steel made of steel. The reinforcement steel bar 1A includes a shaft part 10 that extends in a front-back direction, and a head part 20 formed at a front end portion of the shaft part 10.

The shaft part 10 is formed by providing grid ribs 11 on an outer peripheral surface of a rod-shaped member having a circular cross section. Accordingly, the ribs 11 form asperities on the outer peripheral surface of the shaft part 10.

The head part 20 is formed at the front end portion of the shaft part 10. The head part 20 is a region of the front end portion of the shaft part 10 shaped by forging. In other words, the shaft part 10 and the head part 20 constitute an integrated member.

The head part 20 projects from the shaft part 10 in a radial direction thereof. As shown in FIG. 3, the head part 20 is formed into a substantially triangular shape in front view. The base of the triangle is located at an upper end portion of the head part 20 while the vertex of the triangle is located at a lower end portion of the head part 20. The head part 20 has a bilaterally symmetric shape.

As shown in FIGS. 1 and 3, the head part 20 is provided with a front end surface 21, a base end surface 22, an upper end surface 23, a left side surface 24, and a right side surface 25.

As shown in FIG. 3, a width L1 in a right-left direction of the upper end portion of the head part 20 is formed wider than a diameter D of the shaft part 10. Meanwhile, the width in the right-left direction of the lower end portion of the head part 20 is formed narrower than the diameter D of the shaft part 10.

The width L1 in the right-left direction of the upper end portion of the head part 20 is set preferably in a range from 1.9 to 2.5 times as large as the diameter D of the shaft part 10.

The upper end surface 23 extending parallel to an axial direction of the shaft part 10 is formed at the upper end portion of the head part 20. The upper end surface 23 is slightly inclined downward from a central portion in the right-left direction to two side edge portions. In other words, the upper end surface 23 is a convex surface in which the center point in the right-left direction is the highest while the surface gradually declines from the central portion toward the right and left side edge portions. The upper end surface 23 is inclined preferably at an angle equal to or below 8 degrees with respect to a horizontal direction.

A distance L2 from the shaft center (the axis) of the shaft part 10 to the central portion in the right-left direction of the upper end surface 23 is set preferably in a range from 0.5 to 0.7 times as large as the diameter of the shaft part 10. This makes it possible to hold the distance L2 within a range of a manufacturing error when forging the head part 20.

The left side surface 24 and the right side surface 25 are flat surfaces that extend downward from the left and right edge parts of the upper end surface 23.

The left side surface 24 and the right side surface 25 are inclined such that the width in the right-left direction of the head part 20 is gradually reduced from the upper end portion to the lower end portion thereof. That is to say, an interval in the right-left direction between the left side surface 24 and the right side surface 25 is gradually reduced downward, and a lower edge part of the left side surface 24 comes into contact with a lower edge part of the right side surface 25.

An opening angle R between the left side surface 24 and the right side surface 25 is set preferably in a range from about 55 degrees to 65 degrees.

The front end surface 21 of the head part 20 is a substantially triangular surface in front view, and is segmented into an upper front end surface 21 a and a lower front end surface 21 b.

As shown in FIG. 2, the upper front end surface 21 a is a flat surface with its normal direction aligned with the axial direction of the shaft part 10. As shown in FIG. 3, the upper front end surface 21 a is formed into a trapezoidal shape, in which a lower bottom is formed shorter than an upper bottom.

An outer edge portion of the upper front end surface 21 a is located outside, in the radial direction of the shaft part 10, of a corner portion 26 between the base end surface 22 (see FIG. 2) of the head part 20 and the outer peripheral surface of the shaft part 10.

As shown in FIG. 2, the corner portion 26 is formed into a curved surface. The corner portion 26 is preferably formed into the curved surface having a radius in a range from 1.0 mm to 3.0 mm.

As shown in FIG. 3, the lower front end surface 21 b is continuously formed below the upper front end surface 21 a. The lower front end surface 21 b is a triangular flat surface with its base disposed on an upper side. As shown in FIG. 2, the lower front end surface 21 b is inclined in such a way as to be displaced backward as the surface goes down.

A thickness L3 between the upper front end surface 21 a and the base end surface 22 is set preferably in a range from 1.0 to 1.2 times as large as the diameter D of the shaft part 10. Meanwhile, a thickness L4 between a lower edge portion of the lower front end surface 21 b and the base end surface 22 is set preferably in a range from 0.4 to 0.7 times as large as the diameter D of the shaft part 10.

As shown in FIG. 1, a protrusion 27 projects from the upper front end surface 21 a of the first embodiment. The protrusion 27 is an elongated region with its axial cross section formed into a rectangular shape. As shown in FIG. 3, the protrusion 27 extends straight in the right-left direction while passing through the center point of the shaft part 10. An amount of projection of the protrusion 27 is set preferably in a range from 0.5 mm to 2.0 mm.

Next, a coupling structure for decks 110 by using the reinforcement steel bar 1A of the first embodiment will be described.

As shown in FIG. 4, the first embodiment will describe a coupling structure for coupling the decks 110 laid on a bridge superstructure 100 that includes RC decks.

The decks 110 that are adjacent to each other are placed on bridge beams with an interval in between. Thus, a space 200 is defined between the adjacent decks 110.

Each deck 110 is a precast member made of ferroconcrete. The reinforcement steel bar 1A of the first embodiment is laid inside the deck 110. Moreover, a region on the front end side of the reinforcement steel bar 1A projects in a horizontal direction from an end surface of the deck 110.

Meanwhile, other reinforcement steel bars 2 are disposed between the reinforcement steel bar 1A projecting from one of the decks 110 and the reinforcement steel bar 1A projecting from the other deck 110.

Regarding the reinforcement steel bar 1A on the upper side in FIG. 4, the head part 20 is disposed such that the upper end surface 23 is directed to an upper surface of concrete C. Meanwhile, regarding the reinforcement steel bar 1A on the lower side in FIG. 4, the head part 20 is disposed such that the upper end surface 23 is directed to a lower surface of the concrete C.

After the reinforcement steel bars 1A are laid in the space 200 as described above, concrete C is placed in the space 200 to bury the reinforcement steel bars 1A in the concrete C.

Then, the reinforcement steel bars 1A in the decks 110 are anchored to the concrete C, whereby the adjacent decks 110 are coupled to each other through the intermediary of the concrete C.

According to the reinforcement steel bar 1A of the first embodiment described above, when a stress attributable to a bending tensile force and to a punching shear force is applied to the reinforcement steel bar 1A buried in the concrete C, the head part 20 as well as the ribs 11 on the shaft part 10 engage with the concrete C.

Meanwhile, in the reinforcement steel bar 1A of the first embodiment, a surface area of the head part 20 is increased by the protrusion 27 provided on the front end surface 21 of the head part 20.

As a consequence, the reinforcement steel bar 1A of the first embodiment can increase an anchoring force to the concrete C.

In the reinforcement steel bar 1A of the first embodiment, the head part 20 projects toward the other reinforcement steel bars 2. Accordingly, even if the reinforcement steel bar 1A moves inside the concrete C, it is possible to suppress a displacement of the reinforcement steel bar 1A by allowing the head part 20 to get stuck with the other reinforcement steel bars 2.

As shown in FIG. 2, in the reinforcement steel bar 1A of the first embodiment, the thickness L3 from the upper front end surface 21 a to the corner portion 26 between the base end surface 22 of the head part 20 and the outer peripheral surface of the shaft part 10 is equal to a maximum value of the thickness of the head part 20 in the axial direction of the shaft part 10. As shown in FIG. 4, this makes it possible to increase shear strength of the head part 20 when a tensile force is applied from the concrete C to the reinforcement steel bar 1A.

In the reinforcement steel bar 1A of the first embodiment, the corner portion 26 between the base end surface 22 of the head part 20 and the outer peripheral surface of the shaft part 10 is formed into the curved surface (see FIG. 2). For this reason, when the pressure originating from the concrete C is applied from the base end side to the front end side of the reinforcement steel bar 1A, a stress is less likely to be concentrated on the corner portion 26 between the base end surface 22 of the head part 20 and the outer peripheral surface of the shaft part 10. This makes it possible to increase fatigue resistance at a junction between the head part 20 and the shaft part 10.

When the reinforcement steel bar 1A of the first embodiment is laid in the concrete C, if the upper end surface 23 of the head part 20 is directed to the lower surface of the concrete C, a covering depth of the upper end surface 23 of the head part 20 is subject to regulation.

Moreover, a covering depth T1 of the upper end surface 23 of the head part 20 becomes substantially the same as a covering depth T2 of the shaft part 10. Thus, it is possible to control the covering depth of the entire reinforcement steel bar 1A.

In this way, it is possible to reduce the weight of the superstructure 100 by controlling the covering depth of the entire reinforcement steel bar 1A. As described above, when the reinforcement steel bar 1A of the first embodiment is applied to the deck 110, it is possible to increase strength of the deck 110 while reducing its weight as low as that of the existing deck. Furthermore, the thickness of the deck 110 applying the reinforcement steel bar 1A can be kept at a minimum deck thickness as defined in design standards (the Specifications for Highway Bridges).

As shown in FIG. 3, in the reinforcement steel bar 1A of the first embodiment, the upper end surface 23 of the head part 20 is inclined downward from the central portion in the right-left direction to the side edge portions. In this way, as shown in FIG. 4, if the upper end surface 23 of the head part 20 is slightly tilted about the axis of the shaft part 10 when the reinforcement steel bar 1A is laid in the concrete C, it is still possible to control the covering depth T1 of the upper end surface 23 of the head part 20.

As shown in FIG. 3, in the reinforcement steel bar 1A of the first embodiment, the head part 20 is formed into the substantially triangular shape. Thus, a volume of the head part 20 can be reduced more than that in a configuration of forming the head part 20 into a semicircular shape. This makes it possible to reduce an amount of deformation of the end portion of the shaft part 10 so as not to cut off metallic fibers (fiber flows) of the head part 20 when the head part 20 is formed by forging the end portion of the shaft part 10.

Moreover, in the reinforcement steel bar 1A of the first embodiment, each of the left side surface 24 and the right side surface 25 of the head part 20 is formed into the flat surface.

Accordingly, in the reinforcement steel bar 1A of the first embodiment, the head part 20 can be easily formed at the end portion of the shaft part 10 by forging.

Although the first embodiment of the present invention has been described above, the present invention is not limited to the first embodiment but can be changed as appropriate within a range not departing from the scope thereof.

In the first embodiment, the upper end surface 23 of the head part 20 is inclined as shown in FIG. 3. Instead, the entire upper end surface 23 of the head part 20 may be formed into a flat surface as shown in FIG. 5.

In the first embodiment, each of the left side surface 24 and the right side surface 25 of the head part 20 is formed into the flat surface as shown in FIG. 3. Instead, the left side surface 24 and the right side surface 25 may be curved to form an outwardly convex shape as shown in FIG. 6. Alternatively, the left side surface 24 and the right side surface 25 may be curved to form an inwardly concave shape.

In the first embodiment, the protrusion 27 is formed on the front end surface 21 of the head part 20 as shown in FIG. 3. However, the region on an outer surface of the head part 20 to be provided with the protrusion 27 is not limited to a particular region. For example, the protrusion 27 may also be formed on the left side surface 24 and the right side surface 25 of the head part 20. Alternatively, the protrusion 27 need not be formed on the outer surface of the head part 20.

In the first embodiment, the ribs 11 are formed on the outer peripheral surface of the shaft part 10 as shown in FIG. 1. However, the ribs 11 need not be formed on the outer peripheral surface of the shaft part 10. That is to say, the shaft part 10 may be formed of a round rod.

While the first embodiment has described the structure for coupling the decks 110 to each other as shown in FIG. 4, the structure that can apply the reinforcement steel of the present invention is not limited thereto, and the present invention is applicable to various ferroconcrete structures.

In the first embodiment, the reinforcement steel bars 1A are laid in a direction of extension of the superstructure 100. Instead, the reinforcement steel bars 1A may be laid in a width direction of the superstructure 100 so as to connect decks that are juxtaposed in the width direction of the superstructure 100. Moreover, the layout structure of the reinforcement steel bars 1A including orientations, positions, and the like thereof are not limited.

Second Embodiment

Next, a reinforcement steel bar 1B of a second embodiment will be described.

As shown in FIG. 7, the reinforcement steel bar 1B of the second embodiment has substantially the same configuration as that of the reinforcement steel bar 1A of the above-described first embodiment (see FIG. 1), except that the head part 20 is further provided with a flange portion 28.

In the reinforcement steel bar 1B of the second embodiment, the plate-shaped flange portion 28 is formed at an outer peripheral part of abase end portion of the head part 20. The flange portion 28 projects outward from the left side surface 24 and the right side surface 25 of the head part 20. As shown in FIG. 8, each of a front end surface and a base end surface of the flange portion 28 is a flat surface with its normal direction aligned with the axial direction of the shaft part 10.

A width of the flange portion 28 in a direction of projection is set preferably in a range from 0.1 to 0.5 times as large as the diameter of the shaft part 10. In the meantime, as shown in FIG. 8, a thickness L5 of the flange portion 28 in the axial direction of the shaft part 10 is set preferably in a range from 0.4 to 0.7 times as large as the diameter of the shaft part 10.

Here, the flange portion 28 is formed on the left side surface 24 and the right side surface 25 of the head part 20 in the second embodiment. Instead, the flange portion 28 may be formed entirely around the head part 20.

Meanwhile, in the second embodiment, the protrusion 27 is formed on the front end surface 21, the left side surface 24, and the right side surface 25 of the head part 20, as well as on a front end surface of the flange portion 28 thereof as shown in FIGS. 7 and 9.

As shown in FIG. 8, according to the above-described reinforcement steel bar 1B of the second embodiment, when the pressure originating from the concrete is applied in the axial direction of the reinforcement steel bar 1B, the reinforcement steel bar 1B can receive the pressure by using the flange portion 28 in addition to any of the front end surface 21 and the base end surface 22 of the head part 20. Thus, the reinforcement steel bar 1B of the second embodiment can increase the anchoring force to the concrete.

Meanwhile, in the reinforcement steel bar 1B of the second embodiment, its anchoring force to the concrete is increased by providing the head part 20 with the flange portion 28. Thus, the volume of the head part 20 can be reduced more than that in a configuration of not providing the head part 20 with the flange portion 28. This makes it possible to reduce an amount of deformation of the end portion of the shaft part 10 when the head part 20 is formed by forging the end portion of the shaft part 10.

Although the second embodiment of the present invention has been described above, the present invention is not limited to the second embodiment but can be changed as appropriate within the range not departing from the scope thereof as has been mentioned in regard to the first embodiment.

For example, the upper end surface 23 of the head part 20 may be formed into a flat surface as shown in FIG. 10. Meanwhile, each of the left side surface 24 and the right side surface 25 of the head part 20 may be curved to form an outwardly convex shape as shown in FIG. 11. On the other hand, the left side surface 24 and the right side surface 25 may be curved to form an inwardly concave shape.

Meanwhile, in the second embodiment, the region on the outer surface of the head part 20 to be provided with the protrusion 27 is not limited to a particular region. For example, the protrusion 27 may be formed only on the front end surface 21 of the head part 20. Alternatively, the protrusion 27 need not be formed on the outer surface of the head part 20.

Third Embodiment

Next, a reinforcement steel bar 1C of a third embodiment will be described.

As shown in FIG. 12, the reinforcement steel bar 1C of the third embodiment has substantially the same configuration as that of the reinforcement steel bar 1A of the first embodiment (see FIG. 1), except that structures of the junction between the head part 20 and the shaft part 10 are different from each other. In the reinforcement steel bar 1C of the third embodiment, a recess 29 is formed at the junction between the head part 20 and the shaft part 10.

In the third embodiment, the recess 29 is formed along the corner portion 26 between the base end surface 22 of the head part 20 and the outer peripheral surface of the shaft part 10.

The recess 29 is a region of the base end surface 22 recessed along an outer peripheral edge portion of the shaft part 10. A bottom surface of the recess 29 is formed into a curved surface.

In the third embodiment, the recess 29 is formed in the base end surface 22 at the time of forging the front end portion of the shaft part 10 to the head part 20.

Note that the method of forming the recess 29 in the base end surface 22 is not limited to a particular method. However, when the recess 29 is formed at the timing of forging, it is possible to retain the strength of the head part 20 because the metallic fibers (the fiber flows) of the head part 20 are not cut off in this case.

Although the third embodiment of the present invention has been described above, the present invention is not limited to the third embodiment but can be changed as appropriate within the range not departing from the scope thereof as has been mentioned in regard to the first embodiment.

In the third embodiment, the recess 29 is formed continuously into a circular shape along the outer peripheral surface of the shaft part 10 as shown in FIG. 12. However, the width and depth of the recess 29 are not limited to particular values. Meanwhile, the recess 29 may be formed discontinuously instead.

In the meantime, the bottom surface of the recess 29 of the third embodiment is formed into the curved surface. However, the shape of the recess 29 is not limited to a particular shape, and its cross section may be rectangular or triangular. Alternatively, the bottom surface of the recess 29 may have a curved surface formed by continuously providing multiple curved surfaces with different curvatures. 

1. A reinforcement steel comprising: a shaft part extending in a front-back direction; and a forged head part provided at an end portion of the shaft part, wherein a width in a right-left direction of an upper end portion of the head part is formed wider than a diameter of the shaft part, the width in the right-left direction of a lower end portion of the head part is formed narrower than the diameter of the shaft part, an end surface extending parallel to an axial direction of the shaft part is formed at the upper end portion of the head part, and a left side surface and a right side surface of the head part are inclined such that the width in the right-left direction of the head part is gradually reduced from the upper end portion to the lower end portion of the head part, and the reinforcement steel is provided with grid ribs on an outer peripheral surface of the shaft part.
 2. The reinforcement steel according to claim 1, wherein each of the left side surface and the right side surface is a flat surface.
 3. The reinforcement steel according claim 1, wherein a plate-shaped flange portion is formed at an outer peripheral part of a base end portion of the head part.
 4. The reinforcement steel according claim 1, wherein a linearly extending protrusion projects from an outer surface of the head part.
 5. The reinforcement steel according claim 1, wherein the end surface is inclined downward from a central portion in the right-left direction to a side edge portion.
 6. The reinforcement steel according claim 1, wherein a front end surface is formed at a front end portion of the head part, and a normal line of the front end surface is parallel to the axial direction of the shaft part, and an outer edge portion of the front end surface is outer than a corner portion in a radial direction of the shaft part, the corner portion is provided between a base end surface of the head part and an outer peripheral surface of the shaft part.
 7. The reinforcement steel according claim 6, wherein the corner portion is formed into a curved surface.
 8. The reinforcement steel according claim 6, wherein a recess is formed along the corner portion.
 9. (canceled) 